Diversity is a well know method for providing improved system gain in radio applications involving multipath fading, such as land mobile radio applications. Frequency diversity is one type of diversity improvement in which the same information is transmitted on two or more RF (Radio Frequency) channels, separated far enough in frequency such that the fading is uncorrelated. For current typical land mobile radio systems, approximately 100 kHz or more of separation is needed. The system gain which can be obtained in this way depends on the degree to which the received signals are uncorrelated and the method of combining used. Details of such arrangements are extensively documented in a number of well know texts, including "Microwave Mobile Communications" by W. C. Jakes et al. Gain values of 6 dB to 8 dB are not uncommonly quoted for two-branch systems.
Frequency diversity is advantageous over the alternatives of time or space diversity, in not contributing to increased signal delay (a disadvantage in voice systems) or requiring the installation of extra, physically well separated antennas. The latter can be a problem from several viewpoints including the growing reluctance to disturb the environment and, at lower RF frequencies, the need for spacing dimensions which are incompatible with towers or even building installations.
However, frequency diversity itself has the disadvantage of requiring extra frequency assignment--difficult in spectrally congested markets--and duplication of the transmitter portion of the radio in today's FM radios which use class C amplifiers.
There is a need for improved quality of communication in trunking radio systems within the constraint of limited frequency assignment.